Titanium dioxide (TiO2) is one of the most widely studied semiconductor photocatalysts for a wide array of applications. Currently there is particular interest in uses associated with energy production such as water splitting [references 1, 2] and photochemical solar cells [reference 3]. There are also a number of reports demonstrating that TiO2 photoreduces CO2 to chemical fuels like methane, methanol, carbon monoxide etc and thus, provides a carbon neutral energy production and consumption cycle [references 4-6]. However, the major hurdle in synthesizing titania based photocatalysts for solar fuel generation is to prepare materials that have high reactivity with low recombination rates of charge carriers, and strong visible light absorbance.
There are several approaches to enhance titania's photoefficiency and photoresponse. Based on the electron paramagnetic resonance (EPR) results by Hurum et al., it was proposed that TiO2 with mixed phases of anatase and rutile tends to exhibit higher photoactivity than pure phases alone and has a visible light response [references 7-9]. When anatase and rutile crystals are effectively intermixed with each other, there exist solid-solid interfaces across which photoexcited electrons are transferred from rutile to lower energy anatase lattice trapping sites, thus, hindering the recombination of electrons and holes. Rutile acts as an antenna (because of its light absorption up to about 410 nm) to extend the photo response of the mixed phase catalyst into the visible light region. There are also unique tetrahedrally coordinated Ti4+ trapping sites associated with the solid-solid interfaces of the nano-particles [reference 9]. Mixed phase titania nanocomposites have been synthesized with a high density of solid-solid interfaces by reactive direct current (DC) magnetron sputtering [reference 10]. They showed enhanced photo-activity for both oxidative and reductive chemistry [reference 10] as well as red-shifted photo-response compared to pure phase titania and mixed phase titania synthesized by solvo-thermal methods.
Oxygen vacancies can be created by doping anions such as N or C, or transition metals such as Nb, Al, etc. into TiO2 films [references 12-17] as ways to extend TiO2's photoresponse by introducing intermediate surface states that narrow the band gap. Based on diffuse reflectance spectra (DRS) measurements of powder samples, Serpone et al. [reference 18-20], on the other hand, argued that the dopants at low concentration induce the formation of oxygen vacancies that merely create ‘color centers’ rather than narrowing the band gap by the creation of surface states. While at high concentration, the dopants may be able to narrow the band gap [reference 20]; but the doping materials are also reported to introduce recombination centers for electrons and holes [reference 17, 21], or cause thermal instability problems (mostly for doping metals) [reference 22]. Furthermore, at heavy doping levels the material has extremely different chemical composition and band gap electronic structure.
Oxygen vacancies may also be directly introduced into titania in the absence of dopants during titania formation or after post-deposition annealing or plasma treatment [reference 23]. It is difficult however, to control and finely tune these high energy techniques. Non-stoichiometric titania is reported to display a red shifted photo-response similar to most doped titanias [references 24, 25], and displays both positive and negative influences on photo-reactivity according to the literature. For example, Justicia, et al. [reference 25] stated that a band of defect states existed just below and overlapped with the conduction band minimum, and these states facilitated the transfer of photocarriers to the active sites on the surface. Yates, et al. [reference 23] created bulk and surface oxygen vacancies on stoichiometric rutile (110) and in the subsequent CO2 adsorption experiments, oxygen vacancies on the surface served as adsorption sites. In contrast, Satoshi Takeda, et al., suggested that, oxygen vacancies created energy levels around mid-band gap that served as recombination centers for electrons and holes [reference 26].
Mixed-phase titanium dioxide (TiO2) materials, such as commercially available Degussa P25, show enhanced photoactivity largely due to the synergistic interactions between anatase and rutile phases, which serve to extend their photoresponse to longer wavelengths of light, separate and stabilized charge carriers so as to hinder charge recombination, and create catalytic active sites located at the solid-solid interface. However, improvements in performance are still needed.